Travel time reliability is considered to be one of the key indicators of transport system performance. Knowledge of the mechanisms of travel time unreliability enables the derivation of explanatory models with which travel time reliability can be predicted and utilized in traffic management. Inspired by the macroscopic fundamental diagram (MFD), describing the relationship between production (average flow completing the trips) and vehicle accumulation (average density) in a traffic network, this paper investigates a macroscopic travel time reliability diagram (MRD) relating the travel time reliability to the network accumulation. The MFD relationship has potential because it characterizes the state of an entire traffic network with just two (production and accumulation) or three (adding spatial variability of accumulation) state variables. Likewise, the MRD describes the network travel time reliability as a function of just one independent state variable (network accumulation). Empirical analyses were performed to investigate the variability in MFD as seen in scatters and to show the travel time reliability in relation to the network accumulations. Traffic data from Dutch freeway networks were employed to facilitate the analyses. With the MRD for different freeway networks, it was found that a critical travel time reliability accumulation existed, below which network accumulation had little or no impact on travel time reliability and above which it had a significant impact. Also, the critical travel time reliability accumulation was, in general, lower than the critical MFD accumulation. These findings provide insights for road authorities in how to make trade-offs in traffic management between maximum production and travel time reliability.